The invention relates to a method and an apparatus for the separation of sludge and suspended material from sewage water or other liquid to be purified. More particularly, the invention relates to a biological clarifying apparatus for the treatment of sewage, in which instead of the customary sedimentation of suspended material and the removal thereof in a thickened condition from the bottom of a clarifying chamber, a separation of the suspended material from the purified liquid is effected in a rising stream of the liquid to be purified, the separation being based on the different flow speeds of water and sludge in accordance with the different specific weight of water and sludge preferably aided by a funnel or gutter-like shaping of at least part of the clarifying trough.
In fully biological plants for the clarification of sewage, the sewage is fed to an aeration chamber by free fall or by means of a raw water pumping arrangement, after passing a sand trap and possibly after being preliminarily cleaned mechanically.
In the aeration chamber, aeration takes place with any desired aeration devices, such as surface aerating devices, deep lying aeration devices and so on. Microbes present in the aeration chamber feed on the organic impurities of the sewage and consequently purify it.
In the subsequent clarifying arrangement, the microbes, so-called active slurry, are separated from the water and are completely or partly fed back to the aeration chamber in order that the active slurry concentration is maintained. Purified water which is free of the active slurry can be obtained after return of the active slurry to the aeration chamber.
The separation of the mixture of slurry and water preferably takes place in so-called clarification troughs. Such clarification troughs may have various forms but they all operate in a similar manner, in that the active slurry forms a sediment as a result of its somewhat higher specific weight than the water, and is drawn off in a more of less thickened condition at the bottom of the clarifying trough, and is completely or partly returned to the aeration chamber. Moreover, all of the known arrangements have in common the fact that the separation of active slurry and water takes place in different directions, i.e. the clarified water generally flows upwardly in the case of circular troughs or towards the side in the case of longitudinal troughs, while the active slurry moves downwardly. Separation systems of the kind above described have been known since the beginning of sewage purification technology and although they have become well established they have a number of disadvantages. For example, heavy or bulky material passing into the clarifying trough, more particularly when there is no preliminary mechanical cleaning, collects on the bottom of the clarifying trough, since its sinking speed is greater than that of the active slurry, and can cause blockages there or impair or prevent the necessary feeding back of the active slurry. This causes a reduction in the efficiency of the clarifying trough or even a complete interruption of its effective operation.
A considerable disadvantage is that with many after-clarifying arrangements a large portion of the active slurry remains to long in the unaerated clarifying trough since as a result of its specific weight and the considerable upward current in the lower part of the clarifying trough, it does not come into the range of the means by which it is drawn off. This leads to decay and expanding of the active slurry, and is particularly liable to occur with clarifying troughs of installations of medium and of small size, especially if they are operated with a high volume of slurry.